Apparatus and procedures for indirect extrusion of extrudable material through a die are well known in the art. Such apparatus suffers to some extent from the requirement that after each extrusion stroke the die must be removed from the ram prior to retraction of the ram through the container and then reattached prior to the extrusion stroke. Conventionally, the die has been formed to provide a sealing, sliding fit with the container's inner wall during the extrusion stroke of the die. Yet, because of this sealing fit and due to a build-up of a shell of the material being extruded on the inner wall of the container, movement of the die in the retraction direction through the container is impeded. If the die is not removed from its supporting ram at the end of each extrusion stroke, excessive force is required to carry out complete retraction. The shell of material can cause the die to wedge in the confines of the container during retraction. Damage to either die or container or both may result. Further, retraction of the assembled die and ram requires an increase in the size and strength of the assembly and associated structure.
Where, on the other hand, the die is removed from the ram prior to retraction of the latter, a dead cycle time is produced in the extrusion process. This obviously increases the cost of operation of the equipment and the cost of the products being produced.
The prior art U.S. Pat. Nos. 3,630,064 and 3,522,721 discuss the above problems and set out certain solutions of the same. To this end, both patents disclose a composite die structure. The composite die structure includes a die holder and a die insert received in the holder and having an extruding orifice.
In the structure disclosed in U.S. Pat. No. 3,630,064, the die holder is formed with a first circumferential flange for coupling it to the ram structure. A second, larger circumferential flange serves to provide a seal at the wall of the container during only an extrusion stroke. This second flange remains spaced from the inner wall of the container at all times other than during an extrusion stroke. Elastic radial expansion of the die holder during the extrusion stroke effects the sealing relation with the container. The die holder and insert have beveled surfaces which mate together in a wedging action.
While the structure disclosed in U.S. Pat. No. 3,630,064 provides easy retraction of the assembled die and ram, it suffers from the disadvantage that extrusion material will tend to enter into the gap formed by the mating beveled surfaces of the die insert and the die holder. The material may cause damage to the structure. Also, the die holder tends to become tightly wedged onto the die insert during repeated use tending to permanently enlarge the outer dimension of the die holder and thus decrease the clearance initially designed for permitting easy retraction.
In the structure disclosed in U.S. Pat. No. 3,522,721 the die holder is supported on the end of the ram in encircling relation with the die insert. The die holder is removed after each extrusion stroke and the ram with the attached die insert is retracted through the container. The die holder is annular in shape, completely surrounds the die insert to protect it, and has an inner conical surface adapted to mate with the exterior conical surface of the die insert.
By having the die holder seated against the end of the ram rather than on the die insert, there will be no wedging action of the die holder and die insert as may be produced in the structure of U.S. Pat. No. 3,630,064. However, without any tight precise fit of the mating surfaces of the die holder and die insert and, to some extent, even where there is a very precise fit of these two members, billet material tends to become extruded between the mating surfaces during the extrusion stroke. As indicated above, this can cause damage to the die holder and die insert and can also upset the relationship between the dimensions of the die holder and container wall.